New web app Fuelly tracks your car's mileage per gallon (MPG), shares it with friends and compares it to the mileage of other Fuelly users.Sign up for Fuelly and specify your car's make, model and year. The app will display the EPA's MPG estimates for your car (average, city, and highway). Each time you fill the gas tank, use Fuelly to record the number of gallons you've purchased and the number of miles you've driven since the last fuel-up (an iPhone-friendly version exists, to make it easier).
Fuelly will track your MPG over time, as well as your friends' and other members -- even going so far as to track which user has had the "best tank" on a per model basis.
What's the point, you ask? (After all, can't you just do this on a piece of paper, like you've been doing for decades?) I suppose it's to make saving fuel more fun by collecting and comparing your stats with your friends'.
It's like Facebook for your car. And if you're looking for a nice pick-me-up, you can always check out the neighbor's stats for his Hummer.
UPDATE: Fuelly seems to be down, probably from too much traffic. Stay tuned.